This invention relates to wireless communication systems, and more particularly, to wireless communication systems using multiple antennas at the receiver and/or multiple antennas at the transmitter.
Wireless communication systems in which the receiver knows the characteristics of the channel between the transmit and receive antennas have certain advantages over systems in which the receiver does not know these characteristics. The channel is the transmission path between the transmit and receive antennas. There is a channel between each transmit and receive antenna, and typically each channel has its own channel characteristics. The channel characteristics affect the transmitted signals. Thus, the received signals are the transmitted signals as modified by respective channel characteristics, plus noise.
Knowing the channel characteristics allows the receiver to recreate the transmitted signal, and, thus, makes it easier to decode the received signal. Therefore, in a system where the receiver knows the channel characteristics, a signal can typically be transmitted either at a higher data rate or at lower power level and still have acceptable error rates, as compared to a system a where the receiver does not know the channel characteristics.
The channel characteristics may be determined by transmitting known symbol sequences, which are referred to as training sequences. The essential characteristics of the training sequences are known by both the receiver and the transmitter. The receiver processes the received training sequences to produce estimates of the channel characteristics.
The channel characteristics change over time. Training sequences are transmitted regularly to obtain current channel characteristics. For example, training sequences can be transmitted at the start of each transmission burst, at regular intervals, or continuously, on either a control channel, or traffic channel, or dedicated channel. For example, in typical Code Division Multiple Access (CDMA) systems, training sequences are transmitted continuously on a pilot channel. Due to noise and interference the training sequences may not be received with the desired accuracy. The desired accuracy is one where the obtained training sequences can be used to determine the channel characteristics accurately enough to enable decoding of the information in the received signal with a desired error rate. In some systems, the problem of not receiving the training sequences with the desired accuracy is alleviated by averaging the received signal over time to obtain a more accurate estimate of the channel characteristics.
Averaging the received signal raises a problem in determining channel characteristics when the channel is changing rapidly, such as for example, when a mobile terminal is moving at high speed. When the channel characteristics change rapidly and when the averaging time needed to distinguish the training sequences accurately is long, then a signal that has been modified by channel characteristics that have long since changed is still being used in the average. Thus, the channel characteristics that are obtained at the receiver in such a case are inaccurate. Therefore, the receiver does not accurately know the channel characteristics, which requires a reduction in the data rate of the transmitted signal for it to be decoded with acceptable error rates. The problem in determining channel characteristics when the channel is changing quickly is exacerbated even further in systems that have multiple antennas at the transmitter.
It is desirable for the receiver to know the channel characteristics in systems that use multiple antennas at the transmitter, so-called multi-input systems, and/or multiple antennas at the receiver, so-called multi-output systems. Multi-input and/or multi-output wireless communication systems can achieve dramatically improved capacity compared to single antenna systems, i.e., single antenna to single antenna systems. In multi-input systems the received signal a receive antenna is typically a superposition of each of the transmitted signals as modified by the channel characteristics. Though the transmitted signals interfere with each other, these signals can be processed at the receiver to separate and decode these signals. Typically, in multi-input systems when the channel characteristics are known the transmitted signal can be transmitted at a higher data rate and still be separated out and decoded with an acceptable error rate.
The problem raised by averaging becomes more serious in multiple-input systems. Particularly, in multiple-input systems the averaging needs to be over a longer period of time and therefore it is difficult to determine the channel characteristics when the channel is changing at, or above, a moderate rate, such as, when a mobile terminal is moving at, or above, a moderate rate, such as, for example, at or above about 17 miles/hour. The averaging needs to be over a longer period of time because in multi-input systems a finite amount of transmit power is divided amongst the multiple transmit antennas, each of which is emitting a unique training sequence. This reduces the signal to noise ratio for each of the training sequences. Therefore, the averaging time needs to be increased to receive the training sequences with an acceptable error rate.